Abstract
The performances of infrared absorption spectroscopic gas sensors using a hollow-core photonic band-gap fiber (HC-PBF) as an absorption cell are usually limited by mode interference of the multimode HC-PBF. To minimize the effect of mode interference, a free space coupling structure was proposed for mode transfer from a single mode fiber (SMF) to a HC-PBF and also from the HC-PBF to a photo-detector. The parameters of the free space coupling structure were optimized based on a detailed theoretical analysis and numerical simulation, and the mode inference reduction was experimentally verified. Wavelength modulation spectroscopy (WMS) combined with the second harmonic (2f) detection was utilized in methane (CH4) measurements by targeting an absorption line at 1653 nm. Using a ∼2 mW laser and a 0.45 m-long HC-PBF for CH4 detection at atmospheric pressure, the optimized sensor demonstrated a 1σ detection limit of ∼4.35 parts per million in volume (ppmv) with a 0.5 s averaging time and a minimum detection limit of ∼360 parts per billion in volume (ppbv) with an averaging time of 75 s. The proposed structure can also be used in a HC-PBF-cell based infrared absorption gas sensor system for the detection of any gas species for suppression of mode interference.
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